One of the reasons why Snap and Flatpak were developed was to remove the dependency issues found with traditional package managers. Snap and Flatpak packages contain all the software necessary to install the package in question, including dependencies.
So, when you go to install a certain piece of software via either Snap or Flatpak, you don’t have to worry about installing dependencies because the Snap or Flatpak package has taken care of that issue for you.
I’ve certainly found Flatpak’s to be great when some software updates had been breaking due to dependencies which were at different versions to the one that the software wanted. Also, some devs are preferring to distribute Flatpaks or Snaps as it makes support so much easier, and installing on different distro’s is also a “snap”.
Yes it’s true, I suppose, that proprietary software could also be packaged into a Snap or Flatpak, as the app is not being built from compiling the source code at all.
The linked article also gives a nice comparison between the two formats. Of course, as a user, you can use either, or you can use both if you wish.
See https://www.zdnet.com/article/why-snap-and-flatpak-make-linux-a-better-os-and-how-theyre-different